1. Field of the Invention
This invention relates to an improvement in a delayed coking process.
2. Description of the Prior Art
Delayed coking is a well-known process in which a hydrocarbonaceous oil is heated to a coking temperature and the preheated oil is introduced into a coking drum to produce a vapor phase product, including normally liquid hydrocarbons and coke. The drum is decoked by hydraulic means or by mechanical means. In most configurations of the delayed coking process, the fresh hydrocarbonaceous coker feed is introduced into the coker product fractionator, usually for heat exchange purposes, where it combines with the heavy coker products that are recycled to the coker heater. See Hydrocarbon Processing, September, 1980, pages 153.
It is known that decreasing the recycle ratio of the fractionator bottoms fraction that is recycled to the coking preheater will increase the hydrocarbon liquid yield and decrease the coke yield of the delayed coker. See, for example, "Delayed Coking. Latest Trends" in Hydrocarbon Processing, May 1982, pages 99 to 104, where the effect of recycle ratio to coke yield is shown. As recycle decreases, the cut point of the recycle increases.
All boiling points referred to herein are atmospheric pressure boiling points unless otherwise specified.
The effect of recycle ratio of heavy coker hydrocarbon product and cut point of the fractionator bottoms recycle on coker product yields is shown in Table I, which summarizes the results of delayed coking run A and delayed coking run B, in which the same feed, namely, a vacuum residuum was utilized.
TABLE I ______________________________________ Effect of Cut Point of Bottoms Recycle on Yields in Delayed Coking Run A B ______________________________________ Feed - Vacuum Residuum Gravity, .degree.API at 60.degree. F. 8.6 7.4 Conradson Carbon wt. % 17.5 18.3 Sulfur, wt. % 3.2 3.1 Ash, wt. % 0.035 0.019 Operating Conditions Coil outlet 930 temperature .degree.F. Coke drum pressure, 33 psig .sup.(1) Recycle, wt. % on 14.8 9.8 fresh feed Cut point between Heavy 848 895 Gas Oil and Recycle, .degree.F. Yields on Fresh Feed C.sub.1 -C.sub.2 gas, wt. % 4.92 4.56 C.sub.3 -C.sub.4 gas, vol. % 8.31 8.20 C.sub.5 -cut point 70.91 72.18 liquids, vol. % Coke, wt. % 31.7 31.0 Coke (corrected, 31.7 29.6 wt. %.sup.(2)) ______________________________________ .sup.(1) Recycled heavy coker product. .sup.(2) Coke yield on run B corrected to run A Conradson carbon (i.e., 17.5 wt. %) using the average carbon producing factor, that is, coke yiel to feed Conradson Carbon Residue.
In run A, the cut point of the fractionator bottoms recycle was 848.degree. F. In run B, the cut point was 895.degree. F. As can be seen from table I, run B produced 29.6 weight percent coke, whereas run A produced 31.7 weight percent coke. Thus, when it is desired to minimize coke production in delayed coking and increase the hydrocarbon liquid yield, it is desirable to increase the cut point of the fractionator bottoms recycle stream.
U.S. Pat. No. 2,159,502 discloses a coking process in which a portion of the coking feed is sent to a product fractionator and a portion is sent directly to a coke chamber.
U.S. Pat. No. 4,066,532 discloses a coker feedstock introduced directly into a furnace in an admixture with product fractionator bottoms.
It has now been found that the cut point of the fractionator bottoms fraction that is withdrawn from the fractionator and recycled to the coker preheating zone can be increased when the fresh oil coker feed is split into at least two streams and that these streams are introduced at specified locations in the process.